EDITORS: Graphic attached.

ANN ARBOR—It’s a surprise and a paradox that when the hottest days of summer are just around the corner, Earth is as far away from the sun as it gets all year. In January, when winter’s icy winds blow, we are as close to the sun as we ever get—three million miles closer to it in winter than in the summer, according to University of Michigan astronomer Richard Teske.

“While this sounds strange, it’s the way Nature has arranged it for us,” Teske said. ” Our seasons are caused by the tilt of Earth’s axis of rotation and not by how far we are from the warming fires of the sun. ”

Earth’s distance from the sun constantly varies as our planet cruises a year-long ellipse-shaped orbit around it. Exactly one place on the ellipse is closest to the sun and exactly one other place lies farthest away. ” Earth’s orbit around the sun isn’t strikingly oval, though. If someone handed you a diagram of the orbit, it would be hard to distinguish from a circle without measuring carefully,” Teske explained. ” Our distance from the sun changes from a maximum of 94,500,000 miles, reached during the first week of

This small difference in heat energy is not enough to change our seasons. They are caused by the tilt of Earth’s axis of rotation, which is not connected with the shape of its orbit. ” It is just a coincidence that Earth is farther from the sun at a time when the Northern Hemisphere tilts toward the sun and we’re having summer. In winter, when the Northern Hemisphere tilts away from the sun, we’re close enough to get a little extra, much-needed heat. The orbit’s shape helps to make our northern winters a bit warmer, and our summers a bit cooler, than they might otherwise be,” Teske said.

The elliptical shape of the Earth’s path around the sun and the times when we are closest or farthest away were figured out hundreds of years ago by astronomers using painstaking measurements of how big the sun seems to be. It looks smallest when we are farthest away, biggest when we are close. Careful determination of how the sun’s size changes during a year is translated into a scale diagram of our orbit around it.

“Humanity has a great stake in the near-circularity of Earth’s orbit, which promotes a stable, dependable climate on the planet,” Teske said. ” If the orbit were more elliptical, we’d swing even closer to the sun and then farther from it again, during a single year. This could make for extremes of weather affecting agricultural productivity and perhaps causing significant changes in living conditions. A few scientists have suggested that Earth’s ice ages may have come about when its orbit was temporarily more elongated than it presently is. ”

All of the nine planets’ orbits are elliptical, not because of a peculiar accident, but because the mathematical laws of Nature require it, according to Teske. These laws make it almost impossible for a planet to have a perfectly circular path around the sun. Among the planets, Venus’ orbit is most nearly circular. Its distance from the sun varies by less than a half-million miles during one Venus year.

Distant Pluto has the most eccentric orbit, Teske said. Although it is considered to be the outermost planet of the sun’s family, Pluto’s orbit is elongated enough that at times it ambles closer to the sun than does neighboring Neptune. When it does, Neptune temporarily becomes the outermost planet. That’s the situation right now. Pluto’s path carried it inside Neptune’s in the year 1979. Now slowly moving along its orbit track, Pluto will re-cross Neptune’s path in 1999 to regain its title of most distant planet from the sun. There is no danger of a collision between the two planets.